1. Field of the Invention
The present invention relates to an optical fiber which improves the optical signal-to-noise ratio (hereinafter referred to as OSNR) in a wavelength division multiplexing optical communication system using Raman amplification over a long repeater span, and an optical communication system employing such an optical fiber.
2. Related Background Art
Recently, in optical communication systems, techniques for compensating for waveform distortions resulting from dispersions by using digital signal processing (DSP) such as digital coherent receiver technology have been advancing in receivers. This has rapidly been increasing demands for improving the OSNR rather than dispersion values in optical transmission lines. For increasing the transmission rate without deteriorating the signal quality at a fixed repeater span in a communication system, the OSNR must be raised. For doubling the transmission rate, for example, it is necessary for the OSNR to be increased by twofold (by 3 dB). The OSNR must also be enhanced when increasing the repeater distance while keeping a transmission rate. For example, the OSNR must be raised by 1 dB for elongating the repeater span by 5 km.
Increasing the effective area Aeff and reducing the transmission loss of an optical fiber, serving as a transmission medium, are effective in enhancing the OSNR of an optical communication system. Increasing the effective area Aeff thus can fully suppress generation of nonlinear optical phenomena even when signal light having a high power propagates through the optical fiber. Optical fibers with the increased effective area Aeff can accept greater incident signal light power. As the incident signal light power is greater, the receiving signal light power becomes higher, whereby the OSNR can be enhanced.
When the transmission loss is lower, a greater signal light power can be received on the receiving side even when the incident signal light power is the same. This can also enhance the OSNR.
Japanese Patent No. 4293156 (Document 1) discloses an optical fiber having an effective area Aeff of 110 μm2 or more and a transmission loss of 0.180 dB/km or less. This optical fiber has a depressed cladding type refractive index profile and is constituted by a core, an inner cladding, and an outer cladding sequentially from the optical axis center. In the optical fiber, the core diameter 2a is 11.5 to 23.0 μm, the diameter ratio of the inner cladding to the core 2b/2a is 1.1 to 7, the relative refractive index difference Δ+ of the core with respect to the outer cladding is 0.15% to 0.30%, and the relative refractive index difference Δ− of the inner cladding with respect to the outer cladding is −0.15% to −0.01%.
M. Bigot-Astruc et al., “Trench-Assisted Profiles for Large-Effective-Area Single Mode Fibers”, Mo.4.B.1, ECOC2008 (Document 2) discloses an optical fiber having an effective Aeff of 120 μm2 and a transmission loss of 0.183 dB/km. This optical fiber has a trench type refractive index profile.